Passenger or goods elevators usually have a speed limiting system which comprises at least one cage brake device and a so-termed speed limiter. The purpose of the speed limiting system consists, in the case of occurrence of an impermissibly high speed and/or impermissibly high acceleration of an elevator cage, of bringing the elevator cage to a standstill through braking by means of the cage brake device. The cage brake device is fastened to the elevator cage and comprises a trigger mechanism which is operatively connected with the speed limiter, which is installed in the elevator installation to be non-displaceable. The speed limiter detects the speed and/or acceleration of the elevator cage by way of this operative connection. If the speed limiter detects an impermissibly high speed and/or an impermissibly high acceleration of the elevator cage the speed limiter activates the trigger mechanism of the cage brake device by way of the said operative connection, whereby the elevator cage is braked.
The terms “impermissibly high speed” and “impermissibly high acceleration” are substituted in the following by the term “impermissible movement state”. Moreover, the designation “speed limiter” is used for devices which detect either an impermissibly high speed or an impermissibly high acceleration or detect both an impermissibly high speed and an impermissibly high acceleration.
The operative connection between the trigger mechanism, which moves synchronously with the elevator cage, of the cage brake device and the speed limiter usually comprises a flexible limiter traction means. Wire cables are possibly used as limiter traction means. However, other flexible traction means, for example synthetic fiber cables, belts or link chains, are also usable. The limiter traction means forms a traction means loop which runs around an upper and a lower limiter traction means roller. These limiter traction means rollers are so arranged that a point of the limiter traction means is movable along the entire movement of the elevator cage and parallel thereto. This point of the limiter traction means is connected with the trigger mechanism of the cage brake device so that the movement of the elevator cage is transmitted by way of the limiter traction means to the limiter traction means rollers and to the speed limiter coupled with one of the limiter traction means rollers. The speed limiter comprises means for detection of the speed and/or the acceleration of the limiter traction means and thus the elevator cage. In addition, the speed limiter comprises means for braking the limiter traction means, which on occurrence of an impermissible movement of the limiter traction mean or of the elevator cage brakes the movement of the limiter traction means by limited braking force. This can be effected, for example, in that the speed limiter blocks or brakes one of the traction means rollers around which the limiter traction means runs or in that a brake device, which acts directly on the limiter traction means, of the speed limiter is activated.
After braking of the limiter traction means produced by the speed limiter as a consequence of an impermissible movement state of the elevator cage, the elevator cage and the trigger mechanism, which is connected with the limiter traction means, of the cage brake device continue to move. As a consequence, a traction force builds up in the braked or stationary limiter traction means and acts as a trigger force on a trigger element which forms part of the trigger mechanism of the cage brake device. This trigger force of the limiter traction means causes triggering of the trigger mechanism and thus activation of the cage brake device, whereupon the cage brake device brakes the moved elevator cage and brings it to a stop.
In order to actuate the trigger mechanism a trigger resistance force of the trigger mechanism has to be overcome by the limiter traction means. This trigger resistance force is set so that the accelerations, which in elevator operation arise at the elevator cage, are transmissible by way of the trigger mechanism to the limiter traction means and the speed limiter. Elevator standards and safety specifications often contain prescriptions with respect to the ratio between the resistance trigger force which is to be overcome at the trigger mechanism and the braking force transmissible as a minimum by the speed limiter to the limiter traction means. For example, elevator standard EN 81 requires the braking force transmissible by the speed limiter to the limiter traction means to correspond with at least double the trigger resistance force required for actuation of the trigger mechanism of the cage brake device. In order to help ensure that the trigger mechanism and the cage brake device in a case of an impermissible movement of the elevator cage are in fact also activated, the limiter traction means is thus not to slip through on the limiter traction means roller blocked by the speed limiter or in the activated brake device, which acts on the limiter traction means, of the speed limiter as long as the traction force present in the limiter traction means is less than double the trigger resistance force required for actuation of the trigger mechanism.
In practice it can be difficult and complicated to adduce evidence that under realistic operating conditions the braking force transmissible by the speed limiter to the limiter traction means corresponds with at least double the effectively occurring trigger resistance force of the trigger mechanism of the cage brake device.